This application claims the priority of Japanese Patent Application Nos. 5-276166 filed on Oct. 6, 1993 and 5-352524 filed on Dec. 29, 1993, which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a hydrogen-absorbing alloy electrode used as a negative electrode for metal hydride alkali secondary batteries and to a process for producing the same.
2. Description of the Prior Art
Hydrogen-absorbing alloys capable of absorbing and discharging hydrogen have been developed actively in recent years. Metal hydride alkaline secondary batteries utilizing these hydrogen-absorbing alloys as negative electrode materials have been attracting much attention and expected to become a mainstream in the next generation, since they are lighter and can be provided with higher capacity as compared to lead, nickel-cadmium and like conventional secondary batteries.
Metal hydride alkaline secondary batteries are generally produced by a process which comprises the steps of pulverizing a block-like hydrogen-absorbing alloy into a powder, mixing the hydrogen-absorbing alloy powder with a binder and other ingredients to obtain a paste, applying the paste on a collector or having a collector be filled with the paste and solidifying the paste by heat drying.
However, the above process have the following problems due to, firstly, the presence of a binder in the obtained electrode and, secondly, the hydrogen-absorbing alloy powder being produced by casting and pulverization.
(A) Problems due to the presence of a binder
1 The presence of a binder on the surface of hydrogen-absorbing alloy particles obstructs formation of 3-phase interfaces between the gas (oxygen), liquid (electrolyte) and solid (hydrogen-absorbing alloy particles), thereby preventing the water formation reaction on consumption of oxygen from proceeding smoothly. As a result, the inside pressure of the battery increases and the electrolyte is eventually discharged together with oxygen outwardly from the battery, so that the battery has poor cycle characteristics. Besides, it becomes necessary to charge with a small current and hence the battery has poor rapid chargeability.
2 The presence of a binder between particles of a hydrogen-absorbing alloy decreases the electronic conductivity between the particles, thereby decreasing the degree of activation of the electrode.
3 It is necessary to decrease the amount of a hydrogen-absorbing alloy to fill up the electrode by the volume occupied by the binder, resulting in a low packing density.
(B) Problems due to casting and pulverization process
1 Casting and pulverization requires a long time to cool a hydrogen-absorbing alloy melt, whereby the same elements contained therein tend to flock together and segregation tends to occur. The metal structure thus formed nonuniformly is inferior in corrosion resistance. Then, the surface of the alloy is oxidized (corroded) to form an inert layer, which deteriorates the cycle characteristics.
2 The manufacturing process of negative electrodes becomes complex, since a separate pulverization process is necessary.
In order to solve the problems (A) due to the presence of a binder, a sintering process has become available, which comprises bonding by sintering, without using any binder, hydrogen-absorbing metal particles obtained by the casting and pulverization process.
This process can solve the problems (A) due to the presence of binder, but the problems (B) due to casting and pulverization process remains unsolved. Furthermore, it is difficult to sinter the hydrogen-absorbing alloy particles obtained by casting and pulverization process, since the particles tend to contact with each other while forming "area contact" and hence heat diffuses upon sintering.
Known hydrogen-absorbing alloys for electrodes include those having a pulverized powder form obtained by pulverizing a flake-like or spherical powder mechanically or electrochemically (Japanese Patent Application Laid-open No. 126361/1992) and those of a spherical powder form obtained by gas atomization (Japanese Patent Application Laid-open No. 116361/1991).
However, conventional hydrogen-absorbing alloy electrodes using the pulverized powder or spherical powder alone have suffered the following troubles.
Hydrogen-absorbing alloy electrodes utilizing the pulverized powder alone have the drawback of low packing density, although they have the advantage of having a small electric contact resistance thanks to the alloy particles being contacted with each other while forming, principally, area contact. On the other hand, hydrogen-absorbing alloy electrodes using the spherical powder alone have the drawback of having a large electric contact resistance because of point contact formed by the alloy particles, although they have the advantage of high packing density. If the former, having a low packing density, are used as a negative electrode, the low-rate discharge characteristics will become worse. If the latter, having a large electric contact resistance between alloy particles, are used as a negative electrode, the high-rate discharge characteristics will become worse.